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General => General Chat => Topic started by: Psi on May 19, 2018, 05:29:05 pm

Title: Post a guess on the min starting voltage for this brushed DC motor
Post by: Psi on May 19, 2018, 05:29:05 pm
So i found this old motor in my junk draw and was quite impressed how little voltage is required for it to start moving.
It's a 5-12V DC brushed motor and has no gearing (1:1)

To win respect from your fellow forum users post a guess for the starting voltage.

I'll post a video showing the min voltage in a few days.

(http://www.eevblog.com/forum/chat/post-a-guess-on-the-min-starting-voltage-for-this-brushed-dc-motor/?action=dlattach;attach=436301;image)
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: tautech on May 19, 2018, 05:57:05 pm
And why not: 0.3V
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Gyro on May 19, 2018, 08:15:15 pm
Those Escap coreless motors with precious metal brushes aren't a million miles off being moving coil meter movements - I'll go for 75mV.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Electro Detective on May 19, 2018, 08:29:02 pm
What are the prizes for the winning guess and runner ups?

Rigol/Siglent DSO,

EEVblog meter,

Audiophool Convention tickets  :scared:


No gearing? I'm buying in with 50 microvolts to get that motor cranking btw

Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Psi on May 19, 2018, 09:34:04 pm
I'm buying in with 50 microvolts to get that motor cranking btw

0.00005V ? That's a bit crazy,  Gyro is pretty close
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Ampera on May 20, 2018, 12:47:01 am
50mV

Just from a physics perspective, it wouldn't take much energy to get that small shaft moving. It wouldn't move much, but it just needs to break the friction of the stuff around it.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Brumby on May 20, 2018, 12:50:19 am
I'll throw up a number: 82mV

No particular reason.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: schmitt trigger on May 20, 2018, 02:18:00 am
Me too

94.5 mV

Because why not
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: NiHaoMike on May 20, 2018, 02:30:21 am
0.1337V
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Zero999 on May 20, 2018, 02:48:36 am
There will be some hysteresis. It will probably start moving at 40mV and once moving, will continue to work down to 20mV.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: amyk on May 20, 2018, 02:57:55 am
67mV.

Here's a 120V brushed AC motor starting at less than 9V:
http://www.youtube.com/watch?v=XDzG1XwimjE (http://www.youtube.com/watch?v=XDzG1XwimjE)
That one is more surprising since the brushes on those motors have a lot more friction, being designed for many amps.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Tomorokoshi on May 20, 2018, 03:13:48 am
42 mV.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Gyro on May 20, 2018, 03:53:06 am
.... Gyro is pretty close

Hey, no fair!  :scared:
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Psi on May 20, 2018, 09:55:48 am
.... Gyro is pretty close

Hey, no fair!  :scared:

yeah sorry, that did occur to me, 4 hours after i posted it.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Tomorokoshi on May 20, 2018, 09:59:23 am
.... Gyro is pretty close

Hey, no fair!  :scared:

yeah sorry, that did occur to me, 4 hours after i posted it.

If that's the case, then perhaps a second guess is in order: 88 μV.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: JohnnyMalaria on May 20, 2018, 12:02:33 pm
Zero
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Ampera on May 20, 2018, 01:03:27 pm
Zero

Free energy everybody!
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Brumby on May 20, 2018, 01:18:22 pm
.... Gyro is pretty close

Hey, no fair!  :scared:

yeah sorry, that did occur to me, 4 hours after i posted it.

I think, therefore, that Gyro should be given first acknowledgement for a good answer.  I will admit I based my guess on his figure.


Reminds me of a guessing competition many years ago at a place I once worked - how many grains of rice were in the jar.  I took some measurements of the jar and some grains of rice (of the same type) and did some volume and packing calculations.  Yes, I did get a few eye rolls.

I came up with a number that was far higher than any other guesses, so even with some significant error on my part, I felt confident I was in the right ball park for a win.  Each person's entry consisted of 5 guesses, so I took my calculation as my middle guess and picked two more higher and two more lower, spaced about 1,000 apart.  Someone after me picked numbers halfway between my guesses, so my coverage was now cut in half.  That was annoying.  Nevertheless, it turned out that my coverage included the number the judges came up with - and I won the tea set.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: ez24 on May 20, 2018, 05:30:28 pm
1.55 v
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Electro Detective on May 20, 2018, 07:34:11 pm

So because of the confusion Gyro is in the lead to win the DSO ?   |O

 ;D
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Gyro on May 20, 2018, 10:24:31 pm
Just because you got it horribly wrong.  :P

So there's a DSO up for grabs now? I was just starting to get my hopes up for Brumby's tea set!  Worst case, I'll settle for 'frank admiration'*.  :D

I used one of those motors many years ago (think it was a 12-24V one) as a replacement for a synchronous motor on a belt-drive turntable. I mounted it straight to the chassis (no vibration damping) and fed it with an regulated supply from an LM317 and multi-turn pot. I think it took about 1.5V for 33-1/3 RPM and was perfectly speed stable and silent without any additional feedback - just its own inherent back emf/current draw.


* Think that's a Douglas Adams quote, as in signed "Yours, with frank admiration".
"FORD: Yeah. Where’s Zaphod?
ARTHUR: He… well… he’s in his cabin signing photographs of himself: “To myself with frank admiration.” But why are the Vogons following us?"
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: BrianHG on May 21, 2018, 08:32:39 am
-65mv
Yes, negative...
The title topic says minimum, and everyone here guessed a positive voltage.  :P
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Gregg on May 22, 2018, 05:08:09 am
It depends on how much your power supply overshoots on startup.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: ez24 on May 22, 2018, 06:15:44 am
It depends on how much your power supply overshoots on startup.

I think overshoot counts thus I have a better chance with 1.55v   :-DD
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: james_s on May 23, 2018, 11:19:57 am
I've found most 120V universal motors will run on less than 12V. Not very fast, but they will start up and run. The 240V motor in the air blower in my spa will also run on 12V.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Brumby on May 23, 2018, 11:39:57 am
I forgot to ask .... is this with or without a Batteriser?
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Psi on May 23, 2018, 08:21:42 pm
Gyro  75mV
Electro Detective 0.050mV
TwoOfFive 50mV
Brumby 82mV
schmitt trigger 94.5mV
NiHaoMike  133.7mV
Hero999  40mV
amyk  67mV
Tomorokoshi 42mV
JohnnyMalaria 0mV
ez24 1550mV
BrianHG  -65mV


So, results are in, and im calling it a tie between Gyro (75mV) and amky (67mV).
Mainly because i stuffed up and let the cat out of the bag.

BrianHG gets some points for guessing perfectly 180deg out of phase.

Video
http://www.youtube.com/watch?v=ovYL_rySem4 (http://www.youtube.com/watch?v=ovYL_rySem4)


Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Gyro on May 23, 2018, 09:22:48 pm
Yay, thanks (congrats to amyk and BrianHG). ;D

It's still impressive to see those motors turning so slowly.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Psi on May 24, 2018, 06:08:07 am
Yeah.

Make sure you checkout the end of the video. I included some closeup shots of the commutator rotating.
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Brumby on May 24, 2018, 12:39:22 pm
Dang, I went high .... should have gone low.

Yes, I am using Gyro's figure as my reference.   :-[
Title: Re: Post a guess on the min starting voltage for this brushed DC motor
Post by: Ampera on May 24, 2018, 12:47:48 pm
I'm happy with my guess, I wasn't that far off.